V4L capturing

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Revision as of 23:57, 29 March 2006 by Liontooth (talk | contribs) (Live x264 capture comparison)
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TV Recording applications

Other frame grabbers

Common configuration and control commands

1. v4l2ucp -- universal control panel for v4l2 (available for Debian from Marillat)

2. Command-line control the TV card (v4lctl is a part of the xawtv package)

  • v4lctl -c /dev/video0 list
  • v4lctl -c /dev/video0 bright "60%"
  • v4lctl -c /dev/video0 contrast "55%"

3. Capture the stream

  • streamer (part of the xawtv package):
Usage:
streamer -i 1 -c /dev/video0 -s 320x240 -q -j 80 -f jpeg -n ntsc -b 24 -o /var/www/webcam.jpeg
streamer -i 1 -c /dev/video0 -s 320x240 -q -j 80 -f jpeg -n ntsc -b 24 -o /dev/stdout |uuencode thief.jpeg|sendmail alarm@foo.com
  • videodog:
Usage:
videodog -x 640 -y 480 -w 3 -b 1 -c 65535 -m PAL -q -d /dev/video0 -j -f /var/www/webcam.jpg
This useful tool supports continuously moving (ftp or scp - ssh copy) of jpeg output to remote server. Also allows put in additional text (date time, location), rotating of image.
Usage:
webcam /etc/webcamrc
See webcams for model and driver details

Compression formats

Live x264 capture comparison

The x264 encoder is now of such a high quality that it is possible to compress live tv on the fly with good results. So far I've been unable to find a solution that delivers on all counts: good audio/video synchronization, small file size, efficient encoding (full size and high quality without dropped frames), and a resulting file that streams with VLC. These results, however, may vary locally; the following is a status report from late March 2006 on a Debian amd64 sid with Marillat's multimedia packages.


application a/v sync file size efficiency streaming
ffmpeg middling small good yes
mencoder great small poor no
transcode poor large great yes


Encoding commands used

ffmpeg -threads 2 -vd /dev/video$DEV -r 29.97 -b 800 -s 640x480 -vcodec h264 -qmax 51 \
-me epzs -deinterlace -g 300 -async 1 -ac 2 -acodec mp3 -ab 64 -ar 32000 \
-ad /dev/dsp$DEV -t $TIM -f avi -y $DIR/$FIL.avi
mencoder -tv driver=v4l2:device=/dev/video$DEV:fps=30000/1001:chanlist=us-bcast:\
audiorate=32000:adevice=/dev/dsp$DEV:input=0:amode=1:normid=4:width=512:height=384 \
-ovc x264 -x264encopts threads=2:bitrate=500:subq=2:me=2:frameref=4:8x8dct \
-oac mp3lame -lameopts cbr:br=96 -endpos $TIM -o $DIR/$FIL.avi tv:// > /dev/null
 transcode -x v4l2,v4l2 -M 2 -i /dev/video$DEV -p /dev/dsp$DEV -y ffmpeg -F h264 \
-c 00:$TIM -g 640x480 -f 29.970,4 -u 1024,2 -Q 5 -e 32000,16,2 \
--lame_preset medium -o $DIR/$FIL.avi

Notes

  • ffmpeg
    • a/v sync drifts a bit -- not reliable
    • encoding efficiency is high, but frames are dropped silently
  • transcode
    • audio lags several seconds behind video after an hour
    • there may be ways to enforce better sync -- not fully explored
    • file size can of course be brought down with "-w 800 -b 96" -- effect on sync not tested
    • does a swell job keeping up with frames -- full-size capture with no drops
  • mencoder
    • creates files that don't stream correctly -- we've not got to the bottom of this
    • encoding efficiency is somewhat below ffmpeg and transcode -- cannot capture to full 640x480 size without dropping frames

I tried three different CPUs -- a dual Opteron 240, a dual-core athlon64x2 3800+, and an athlon64-3000+, and (to my surprise) results are comparable on all three.